1. Field of the Invention
The present invention relates to an ionization vacuum device used as a cold cathode ionization vacuum gauge that measures a pressure of residual gas molecules that exist in vacuum vessel by using a vacuum discharge phenomenon in magnetic field.
2. Description of the Related Art
Conventionally, in a cold cathode ionization vacuum gauge using magnetic field, several kV to 7 kV of direct current high voltage is applied basically between an anode and a cathode arranged in a vacuum vessel and a pressure is measured by using the fact that the discharge current is approximately proportional to a pressure inside the vacuum vessel. Since the discharge current becomes weaker and it becomes impossible to sustain discharge when the pressure becomes lower, a structure is taken that a magnetic field is given to a space between two electrodes to extend an electron pass length and thus prevent diffusion of electrons. A vacuum gauge having this structure using the magnetic field is also called a Penning vacuum gauge because Penning put into practical use for the first time in 1937, a two-electrode constitution where the magnetic field exists is called a Penning Cell, and discharge that occurs in this constitution is called a Penning discharge. Further, the vacuum gauge is also called a cold cathode ionization vacuum gauge because it does not use a hot cathode filament.
Penning vacuum gauges described in Patent Document (Japanese Patent No. 314478, Japanese Laid-open Patent Publications No.Hei11-86777 (Patent No. 3750767), No.Hei7-55735 and No.Hei5-290792) are known. According to these basic operating principles, electrons are confined in a magnetic-field-to-electric-field orthogonal space formed by an anode and a cathode by the work of magnetic field, ions are generated by collision of generated electron cloud and gas molecules, and a pressure is measured by measuring ion current.
As described, since the cold cathode ionization vacuum gauge is a method of effectively using electrons generated from gas molecules by discharge, the above-mentioned hot cathode filament is unnecessary, there is no fear of filament burnout. It is preferably used in a field or a production site where stability is required for long period of time.
Further, since various researchs revealed that the cold cathode ionization vacuum gauge had very large pumping speed, proposal of utilizing it as a vacuum pump was also made (Japanese Patent No.314478).
Meanwhile, in recent vacuum devices, electric components formed of organic insulating materials, such as a vacuum motor, a solenoid and a position detector are attached inside a vacuum vessel. Thus, for example, siloxane is emitted from the organic insulating materials such as silicon rubber. Further, phtalic acid, adipic acid or the like that is added as a plasticizer of thermoplastic resin is emitted in vapor from organic mechanical components. They result in contamination of a vacuum gauge and a pump. In the case of using such a cold cathode ionization vacuum gauge and pump, there occurs a problem that discharge stops in about one day in the worst case. When contaminating molecules fly into the cold cathode ionization vacuum gauge, the contaminating molecules are decomposed and ionized by suffering electron impact, and reach a cathode in cations. Contaminating materials (decomposed molecules) that reached the cathode receive electrons into a neutral state, but they are in a radical state. Therefore, they cause polymerization reaction with contaminating materials flying one after another into polymer, and then the polymer is deposited on a cathode surface to form coating through which electricity is hard to pass. This makes cations flying later to flow into the cathode less easily, and thus there occurs a problem of reducing gauge sensitivity, that is, discharge intensity or the like. Further, as a pump, there occurs a problem that sputtering of cathode is suppressed to cause reduction of pumping performance of the pump or increase of gas emission.
Furthermore, if the coating becomes thicker, there occurs a problem that the coating causes stop of discharge or prevention of activation (for start) of discharge in the worst case.